1. Field of the Invention
The present invention generally relates to a sewing system which forms a sewing pattern on a workpiece such as a pair of trousers or a skirt and particularly to such a sewing system which can modify a set of sewing data corresponding to a sewing pattern and form a sewing pattern according to the modified set of sewing data.
2. Related Art Statement
There is known an electronically controlled sewing system including a two-needle sewing machine which has a pair of sewing needles and a pair of thread-loop catchers and which simultaneously forms respective identical bar-tacking patterns on opposite end portions of a belt loop so as to attach the belt loop to a base material such as a pair of trousers or a skirt. A belt is passed through the belt loops attached to the trousers or the skirt. For example, Japanese Utility Model Application laid open for inspection purposes under Publication No. 6(1994)-81481 discloses a two-needle rotary-hook-type sewing machine and a two-needle shuttle-type sewing machine each of which includes a needle bar having a needle support at its lower end; two needles supported by the needle support; a sewing bed; two thread-loop catchers (rotary hooks, or shuttles) which are provided in the sewing bed and which correspond to the two sewing needles, respectively; a main shaft which is connected to the needle bar and which is driven or rotated by a main motor; and a lower shaft which is connected to the loop catchers and to which the rotation of the main shaft is transmitted to drive or rotate the loop catchers.
Each of the above-indicated two-needle sewing machines further includes a workpiece feeding device having a workpiece-feed plate and a plate cam which is connected to the lower shaft and which drives the workpiece-feed plate. The plate cam has a complicated shape which mechanically drives the workpiece-feed plate such that the workpiece supported by the workpiece-feed plate is fed along an X axis and is fed along a Y axis perpendicular to the X axis, independent of the feeding thereof along the X axis. The plate cam mechanically defines the dimensions of a bar-tacking pattern, such as width and length, and the stitch pitch and stitch number of the same. Hence, a reasonable number of plate cams which correspond to different sorts (or sizes) of belt loops, respectively, are prepared in advance. When a current sort of belt loops are changed to a new sort of belt loops, the most appropriate one of the plate cams is newly selected and used to form, on each of the new belt loops, the corresponding bar-tacking pattern which has the most appropriate dimensions, stitch pitch, and stitch number for the each belt loop. However, both the operability of the sewing machines, and the efficiency of sewing of the belt loops are low.